IME it's not that great a hit in most games, maybe 5fps. Admittedly I have super high end hardware so 5fps isn't as large a hit someone might take on low-mid end hardware so YMMV.
A 5fps hit has no meaning. 6fps->1fps is much different than 1005fps->1000fps please use mspf (milliseconds per frame) or at least a percentage of the fps lost.